1. Field of the Invention
Embodiments of this specification relate generally to wireless communication systems and more particularly to a method for estimating frequency and timing information for multiple channel wireless communication systems.
2. Description of the Related Art
Wireless communication systems use frequency allocation to ensure that different RF devices can function in different frequency ranges. In this manner, for example, a user's cell phone does not interfere with other commercial radio systems. These frequency ranges are called bands.
Within a band, the frequency range may be divided into one or more channels. The bands and channels for one form of wireless communication are defined by, for example, the IEEE 802.11 family of standards. In such wireless communication, a transmitter typically transmits data through a channel to one or more receivers.
The IEEE 802.11 standards also define how the data may be configured into data packets that typically include a preamble and a payload. FIG. 1 illustrates an exemplary packet 100 including a preamble 101 that precedes a payload 102. In this case, packet 100 is being sent in a legacy channel that is 20 MHz wide. Preamble 101 includes a plurality of training fields that provide important information for the receipt of packet 100.
Training fields are typically comprised of short and long types. Both short and long training fields are well-defined patterns that a receiver may analyze to estimate coarse and fine timing information as well as coarse and fine frequency information associated with the transmitted data packet. This timing and frequency information allows the receiver to accurately recover the payload from the transmitted data packet.
The IEEE 802.11n draft standard, for example, also describes how a transmitter may transmit data through two channels instead of a single channel to increase the overall effective width of the channel used by the transmitter. A wider channel may advantageously increase the transfer rate of data. The two channels are typically chosen from within a selected band such that they do not overlap and are often referred to as a control channel and an extension channel. Typically, the control channel is a channel closer to the center of the selected band and the extension channel is a channel closer to one of the edges of the band.
In a two-channel transmission, a transmitter transmits a payload through both the control and extension channels. Notably, a duplicated preamble including training fields precedes the payload transmission on both the control and the extension channel. For example, FIG. 2 illustrates an exemplary packet 200 including a preamble 201A (which in sent on the control channel) and a duplicated preamble 201B (which is sent on the extension channel) that precede a payload 202. In this case, the control and extension channels are 20 MHz wide, thereby allowing a payload of 40 MHz wide to be transmitted (note that preambles 201A and 201B are shown as frequency separated for illustration purposes and are typically abutting one another). In one embodiment, the center frequencies of the control and extension channels may be separated by 20 MHz. In another embodiment, the center frequencies of the control and the extension channels may be separated by 25 MHz.
Using preambles 201A and 201B on the control and extension channels, respectively, legacy devices listening to these channels can recognize the control and extension data packets and can advantageously decode, for example, the length of packet 200, thereby allowing such legacy devices to avoid collisions and interference from other wireless communication systems.
As in the case of a single channel system, the short and long training fields on the control and extension channels may be analyzed by a receiver to estimate coarse and fine timing information as well as coarse and fine frequency information associated with the transmitted payload. One possible method to develop the frequency and timing estimates analyzes the training fields from either the control or extension channels since the preambles and therefore the training fields are repeated on both channels. In the case of a two-channel system, the training fields from the control channel may be analyzed to estimate the timing and frequency information for the payload transmitted on both the control and extension channels. This method is advantageously relatively simple to implement. Another advantage is that the training fields of the control channel may be relatively less likely to suffer from interference than are the training fields of the extension channel. For example, within certain bands, such as the 2.4 GHz band, some channels are more likely to overlap and cause interference, especially extension channels which may be located close to the edge of the band.
However, the signal characteristics of the control channel may be different from the signal characteristics of the extension channel. For example, the control channel may be several dB lower in signal strength compared to the extension channel. This lower signal strength may increase the difficulty in determining the frequency and timing information from the training fields for the both channels especially if the receiver only examines the training fields of the control channel.
As the foregoing illustrates, what is needed in the art is an improved method to estimate timing and frequency information for data packets sent through two frequency separated channels particularly through a relatively wider channel comprised of a control channel and an extension channel.